The role of cyclic AMP as a mediator of a variety of hormones is well documented but the role of cyclic GMP is not yet clearly understood. So far the only system that may be mediated by cyclic GMP, appears to be the cholinergic (muscarinic) system. One of the main problems involved in the study of the role of cyclic GMP in hormone action appears to be the transient effect that the hormone produces on the cyclic GMP system and the magnitude of the effect. Most of the tissues appear to contain only 1/100 of cyclic GMP levels as compared to cyclic AMP levels. Moreover, only a small portion of cells in the tissue respond to hormone stimuli and thereby restrict the magnitude of overall response. We have attempted to solve these problems by developing a sensitive method for assay of cyclic GMP and modifying the methods available for cell isolation in order to get a population of cells which are enriched with one type of cell from tissues which consist of numerous cell types.